1. Field of the Invention
The present invention relates to a fabrication method of thin film transistor, more particularly, to a fabrication method of thin film transistor comprising the step of crystallizing the metal catalyst by selectively irradiating laser beam onto metal catalyst, thereby diffusing the metal catalyst in crystallization using super grain silicon method.
2. Description of Related Art
Generally, a polycrystalline silicon layer is used for various purposes of semiconductor layer for thin film transistor since the polycrystalline silicon layer has merits that it has high electric field effect mobility, it is applied to circuits operated at high speed, and it enables CMOS circuit to be constructed. A thin film transistor using the polycrystalline silicon layer is mostly used in active elements of active matrix liquid crystal display (AMLCD) and switching elements and driving elements of organic light emitting diode (OLED).
The polycrystalline silicon layer used in the thin film transistor is fabricated by direct deposition, high temperature thermal annealing or laser annealing. In case of the laser annealing, many alternative technologies are being studied since expensive laser equipment is required although the laser annealing is performed at low temperature, and high electric field effect mobility is obtained by the laser annealing.
At present, a method for crystallizing amorphous silicon using metal is mostly being studied since the method has merits that the amorphous silicon is promptly crystallized at a lower temperature compared with solid phase crystallization. A crystallization method using metal is divided into metal induced crystallization method and metal induced lateral crystallization method. However, the crystallization method using metal also has problems that characteristics of elements for thin film transistor are deteriorated due to metal contamination.
On the other hand, a technology of forming a polycrystalline silicon layer of good quality by controlling concentration of ions of metal through ion injector, thereby performing high temperature annealing, rapid thermal annealing or laser irradiation and a method for crystallizing the thin film by thermal annealing process after depositing a thin film by spin coating a mixture of organic film having viscous property and liquid phase metal on the polycrystalline silicon layer to flatten the surface of polycrystalline silicon layer as metal induced crystallization method are developed to reduce quantity of metal and form a polycrystalline silicon layer of good quality. However, even in case of the crystallization method, there are problems in aspects of scale-up of grain size and uniformity of grains that are the most important in polycrystalline silicon layer.
In order to solve the foregoing problems, a method for manufacturing polycrystalline silicon layer as a crystallization method using cover layer has been developed as disclosed in Korean Patent Laid-open Publication No. 2003-0060403. The method is a method for obtaining a polycrystalline silicon layer using the seed after forming a seed by depositing a metal catalyst layer on the capping layer and diffusing metal catalyst into the amorphous silicon layer through the capping layer using thermal annealing or laser annealing after forming an amorphous silicon layer on a substrate and forming a capping layer on the amorphous silicon layer. The foregoing method has merits that required level or more of metal contamination is prevented since the metal catalyst is diffused through a cover layer. However, the foregoing method also has problems that it is difficult to uniformly control low concentration of metal catalyst and control crystallization starting position, growing direction and grain size.